Tcl_CreateFileHandler arranges for proc to be
invoked in the future whenever I/O becomes possible on a file
or an exceptional condition exists for the file. The file
is indicated by fd, and the conditions of interest
are indicated by mask. For example, if mask
is TCL_READABLE, proc will be called when
the file is readable.
The callback to proc is made by Tcl_DoOneEvent, so
Tcl_CreateFileHandler is only useful in programs that dispatch
events through Tcl_DoOneEvent or through Tcl commands such
as vwait.

Proc should have arguments and result that match the
type Tcl_FileProc:

typedef void Tcl_FileProc(
ClientData clientData,
int mask);

The clientData parameter to proc is a copy
of the clientData
argument given to Tcl_CreateFileHandler when the callback
was created. Typically, clientData points to a data
structure containing application-specific information about
the file. Mask is an integer mask indicating which
of the requested conditions actually exists for the file; it
will contain a subset of the bits in the mask argument
to Tcl_CreateFileHandler.

There may exist only one handler for a given file at a given time.
If Tcl_CreateFileHandler is called when a handler already
exists for fd, then the new callback replaces the information
that was previously recorded.

Tcl_DeleteFileHandler may be called to delete the
file handler for fd; if no handler exists for the
file given by fd then the procedure has no effect.

The purpose of file handlers is to enable an application to respond to
events while waiting for files to become ready for I/O. For this to work
correctly, the application may need to use non-blocking I/O operations on
the files for which handlers are declared. Otherwise the application may
block if it reads or writes too much data; while waiting for the I/O to
complete the application won't be able to service other events. Use
Tcl_SetChannelOption with -blocking to set the channel into
blocking or nonblocking mode as required.

Note that these interfaces are only supported by the Unix
implementation of the Tcl notifier.